1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method which are suitably used in, for example, copiers, electrostatic printing, printers and electrostatic recording.
2. Description of the Related Art
Conventionally, heat roller fixing methods have widely been employed in devices which fix a toner image transferred onto a surface of a paper sheet in image recording apparatuses. The heat roller fixing methods heat and press paper sheet having an unfixed toner image while the paper is being held and conveyed at a nip portion formed between a fixing roller which contains a halogen heater, etc. therein and a pressing roller which presses the fixing roller.
In addition to the heat roller fixing methods, belt fixing methods have been employed. The belt fixing methods heat and press a paper sheet having an unfixed toner image while the paper sheet is being held and conveyed at a nip portion formed between an endless fixing belt and a pressing roller which presses a fixing roller via the fixing belt, where the fixing belt is wound around the fixing roller and a heating roller which contains a halogen heater, etc. therein.
In the belt fixing methods, since the heat capacity of the fixing belt is small, the warm-up time can be shortened, which is advantageous in terms of energy saving.
In these fixing methods, a toner image fused on paper is brought into contact with the fixing roller and/or the fixing belt. Thus, the fixing roller and/or the fixing belt are/is coated with a fluorine-containing resin excellent in releasing property, and a separation claw is used for the separation of a paper sheet.
The separation claw tends to scratch the surfaces of the fixing roller and the fixing belt since it comes into contact with the fixing roller and fixing belt. As a result, streaky images are output, which is considerably problematic.
In the case of monochromatic image forming apparatuses, the fixing roller is generally a roller containing a metal roller and a TEFLON (registered trademark) coating on the metal roller. Thus, even when the separation claw comes into contact with the fixing roller, the fixing roller is hardly scratched to attain a long service life. Thus, in order to prevent winding jam and other unfavorable phenomena, the separation claw has long been used.
However, in the case of color image forming apparatuses, in order to obtain good color developability, the fixing roller used is a roller whose surface layer is a fluorocoated silicone rubber (in general, a PFA tube having about several tens micrometers) or an oil-coated silicone rubber. In this configuration, the surface layer is soft to easily receive scratches, which is problematic.
Since the scratched fixing roller forms unfavorable streaky pattern on the fixed image, a contact means such as the separation claw is hardly used in color image forming apparatuses at present. Instead, non-contact separation is employed in many cases.
The non-contact separation easily causes winding jam, since paper after fixing is wound around the roller when an adhesive force between toner and fixing roller is high. Particularly in color image formation, since several color layers are stacked on top of another to increase adhesive force, winding jam is easily caused.
At present, the following methods are mainly employed for paper separation in color image forming apparatuses; (1) non-contact separation plate methods using separation plates extending in parallel with the longitudinal and width directions of a fixing roller and a fixing belt which have a small gap therebetween (about 0.2 mm to about 1 mm); (2) non-contact separation claw methods using separation claws disposed at predetermined intervals, while a small gap (about 0.2 mm to about 1 mm) is provided between a fixing roller and a fixing belt; (3) self-stripping methods in which paper is automatically peeled off due to the stiffness of the paper and the elasticity of curved regions of a fixing roller and a fixing belt.
However, since a gap is provided between the fixing roller and the fixing belt in all the above methods, a paper sheet is passed through the gap while closely adhering to the fixing roller and the fixing belt, when a thin paper sheet or a paper sheet having small blank at its top end is passed therethrough or when a solid image (e.g., a picture) is passed therethrough. As a result, winding is caused by the closely adhering, or the paper hits against the separation plate or separation claw to cause winding jam, which is problematic.
In view of this, many proposals have been made and employed on blowing air to the paper separation position for assisting the non-contact separation methods (see, for example, Japanese Patent Application Publication (JP-B) No. 57-46794, and Japanese Patent Application Laid-Open (JP-A) Nos. 2005-157179, 2007-199462 and 2008-003277). Blowing air to the paper separation position improves paper separability to prevent winding jam from occurring.
However, these proposals involve severe surface roughness of an image, especially a solid image, not attaining improvement in both paper separability and solid image quality.
Therefore, at present, demand has arisen for an image forming apparatus and an image forming method which can improve paper separability by air separation to prevent winding jam from occurring, which can prevent glossiness unevenness and solid image's surface roughness, and which can form high-quality images.